How can I change default dialog button text color in android 5

You can try to create the AlertDialog object first, and then use it to set up to change the color of the button and then show it. (Note that on builder object instead of calling show() we call create() to get the AlertDialog object:

//1. create a dialog object 'dialog'
MyCustomDialog builder = new MyCustomDialog(getActivity(), "Try Again", errorMessage); 
AlertDialog dialog = builder.setNegativeButton("OK", new DialogInterface.OnClickListener() {

                @Override
                public void onClick(DialogInterface dialogInterface, int i) {
                    ...
                }

            }).create();

//2. now setup to change color of the button
dialog.setOnShowListener( new OnShowListener() {
    @Override
    public void onShow(DialogInterface arg0) {
        dialog.getButton(AlertDialog.BUTTON_NEGATIVE).setTextColor(COLOR_I_WANT);
    }
});

dialog.show()

The reason you have to do it on onShow() and cannot just get that button after you create your dialog is that the button would not have been created yet.

I changed AlertDialog.BUTTON_POSITIVE to AlertDialog.BUTTON_NEGATIVE to reflect the change in your question. Although it is odd that "OK" button would be a negative button. Usually it is the positive button.

The color of the buttons and other text can also be changed via theme:

values-21/styles.xml

<style name="AppTheme" parent="...">
  ...
  <item name="android:timePickerDialogTheme">@style/AlertDialogCustom</item>
  <item name="android:datePickerDialogTheme">@style/AlertDialogCustom</item>
  <item name="android:alertDialogTheme">@style/AlertDialogCustom</item>
</style>

<style name="AlertDialogCustom" parent="android:Theme.Material.Light.Dialog.Alert">
  <item name="android:colorPrimary">#00397F</item>
  <item name="android:colorAccent">#0AAEEF</item>
</style>

The result:

The simpliest solution is:

dialog.show(); //Only after .show() was called
dialog.getButton(AlertDialog.BUTTON_NEGATIVE).setTextColor(neededColor);
dialog.getButton(AlertDialog.BUTTON_POSITIVE).setTextColor(neededColor);

Here's a natural way to do it with styles:

If your AppTheme is inherited from Theme.MaterialComponents, then:

<style name="AlertDialogTheme" parent="ThemeOverlay.MaterialComponents.Dialog.Alert">
    <item name="buttonBarNegativeButtonStyle">@style/NegativeButtonStyle</item>
    <item name="buttonBarPositiveButtonStyle">@style/PositiveButtonStyle</item>
</style>

<style name="NegativeButtonStyle" parent="Widget.MaterialComponents.Button.TextButton.Dialog">
    <item name="android:textColor">#f00</item>
</style>

<style name="PositiveButtonStyle" parent="Widget.MaterialComponents.Button.TextButton.Dialog">
    <item name="android:textColor">#00f</item>
</style>

If your AppTheme is inherited from Theme.AppCompat:

<style name="AlertDialogTheme" parent="ThemeOverlay.AppCompat.Dialog.Alert">
    <item name="buttonBarNegativeButtonStyle">@style/NegativeButtonStyle</item>
    <item name="buttonBarPositiveButtonStyle">@style/PositiveButtonStyle</item>
</style>

<style name="NegativeButtonStyle" parent="Widget.AppCompat.Button.ButtonBar.AlertDialog">
    <item name="android:textColor">#f00</item>
</style>

<style name="PositiveButtonStyle" parent="Widget.AppCompat.Button.ButtonBar.AlertDialog">
    <item name="android:textColor">#00f</item>
</style>

Use your AlertDialogTheme in your AppTheme

<item name="alertDialogTheme">@style/AlertDialogTheme</item>

or in constructor

androidx.appcompat.app.AlertDialog.Builder(context, R.style.AlertDialogTheme)

or If you are using MaterialAlertDialogBuilder then use

<item name="materialAlertDialogTheme">@style/AlertDialogTheme</item>